Signal filtering system having adaptively cascaded filter stages for developing a variable bandwidth frequency characteristic

Abstract

An adaptive filtering system is disclosed. The system is used in a digital television receiver to extract low frequency vertical detail information from a comb filtered chrominance signal. This system provides wide bandwidth vertical detail information when the amplitude of the chrominance component of the composite video signal is low and narrower bandwidth vertical detail signals as the amplitude of the chrominance component increases. Adaptive filtering is accomplished by passing the comb filtered chrominance signal through a first low-pass filter, developing two reduced amplitude replicas of the filtered signal which are mutually supplementary. The ratio of the amplitudes of these two replicas is variable with a control signal. One replica is passed through a second low-pass filter and combined with the other replica to produce a vertical detail signal. The embodiment of the system that is disclosed includes means for developing a control signal that is proportional to the instantaneous amplitude of the chrominance component of the composite video signal.

What is claimed is: 
     
       1. Apparatus for processing a composite signal having a first signal component, occupying a first band of frequencies, and a second signal component, occupying a second band of frequencies, including a filtering system comprising: a first filter responsive to said composite signal and having a predetermined frequency response characteristic for passing signals occupying said first band of frequencies to the relative exclusion of signals occupying said second band of frequencies;   control means coupled to receive said composite signal and responsive to a predetermined band of frequencies of at least one of said first and second signal components, for developing a variable control signal that is proportional to the magnitude of at least one of said signal components;   signal splitting means coupled to said first filter and having first and second output terminals for providing respective signals at said first and second output terminals which are mutually supplementary scaled replicas of the signals provided by said first filter, the amplitudes of said respective scaled replica signals being in a ratio porportional to the instantaneous value of said control signal;   a second filter coupled to the first output terminal of said signal splitting means and having a frequency response characteristic substantially equal to said predetermined frequency response characteristic for passing signals occupying said first band of frequencies to the relative exclusion of signals occupying said second band of frequencies; and   signal combining means coupled to said second filter and to the second output terminal of said signal splitting means for combining the respective signals provided thereby to produce a filtered output signal.   
     
     
       2. Apparatus for processing a composite signal having a first signal component, occupying a first band of frequencies and a second signal component, occupying a second band of frequencies, including a filtering system comprising: a first filter responsive to said composite signal for passing signals occupying said first band of frequencies to the relative exclusion of signals occupying said second band of frequencies;   control means coupled to receive said composite signal and responsive to a predetermined band of frequencies of at least one of said first and second signal components, for developing a control signal representing the magnitude of at least one of said signal components;   attenuating means coupled to said first filter and responsive to said control signal for providing a replica of the signal provided by said first filter reduced in amplitude by an amount proportional to the magnitude of said control signal;   subtracting means coupled to said first filter and to said attenuating means for providing a signal proportional to the difference between the signal provided by said first filter and the signal provided by said attenuating means;   a second filter coupled to said attenuating means and responsive to the signals provided thereby for passing signals occupying said band of frequencies to the relative exclusion of signals occupying said second band of frequencies; and   signal combining means coupled to said second filter and to said subtracting means for combining the respective signals provided thereby to produce a filtered output signal.   
     
     
       3. The apparatus set forth in claim 2 wherein said control means comprises: a third filter for passing the signals occupying the predetermined band of frequencies of at least one of said first and second signal components; and   signal processing means coupled to said third filter for developing said control signal representing the the amplitude of the signals provided by said third filter.   
     
     
       4. The apparatus set forth in claim 2 wherein the frequency characteristic of said second filter is substantially the same as the frequency characteristic of said first filter. 
     
     
       5. In a video signal processing system having a source of comb filtered chrominance signals which include relatively low frequency luminance signal components and relatively high frequency chrominance signal components including two quadrature phase related color difference signal components, apparatus comprising: a first low-pass filter coupled to said source for providing the luminance signal components of said comb filtered chrominance signals to the relative exclusion of said chrominance signal components;   control means coupled to said source and responsive to the chrominance signal components of said comb filtered chrominance signals for developing a control signal proportional to the magnitude of said chrominance signal components;   signal splitting means coupled to said first filter and having first and second output terminals, said signal splitting means providing signals at its first and second output terminals which are mutually supplementary scaled replicas of the signals provided by said first filter, the amplitudes of said scaled replicas being in a ratio proportional to the instantaneous value of said control signal;   a second low-pass filter coupled to the first output terminal of said signal splitting means for further attenuating the chrominance signal components relative to the luminance signal components of the signal provided by said signal splitting means; and   signal combining means coupled to said second filter and to the second output terminal of said signal splitting means for adding the signals provided thereby to produce a filtered output signal.   
     
     
       6. The apparatus set forth in claim 5 wherein said signal splitting means comprises: attenuating means coupled to said first filter and responsive to said control signal for providing, at said first output terminal, a replica of the signal provided by said first filter reduced in amplitude by an amount proportional to the magnitude of said control signal; and   subtracting means coupled to said first filter and to said attenuating means for providing, at said second output terminal, a signal proportional to the difference between the signal provided by said first filter and the signal provided by said attenuating means.   
     
     
       7. The apparatus set forth in claim 6 wherein said control means comprises: a high-pass filter for providing the chrominance signal components to the relative exclusion of the luminance signal components of said comb filtered chrominance signals;   means coupled to said high-pass filter for separating the two color difference signal components from said chrominance signal components; and   means responsive to said two color difference signal components for providing a control signal proportional to the greater of the instantaneous magnitude values of said two color difference signal components.   
     
     
       8. The apparatus set forth in claim 6 wherein the frequency characteristic of said first low-pass filter is substantially the same as the frequency characteristic of said second low-pass filter. 
     
     
       9. In a digital television receiver including a source of samples representing composite video signals which have luminance signal and chrominance signal components including two quadrature phase related color difference signal components, and a comb filter coupled to said source for providing samples representing a comb filtered luminance signal and samples representing a comb filtered chrominance signal in response to said composite video samples, said comb filtered chrominance samples including said chrominance signal components and relatively low frequency luminance signal components, apparatus comprising: a first low-pass filter responsive to said comb filtered chrominance samples for providing samples representing the low frequency luminance signal components thereof to the relative exclusion of said chrominance signal components;   control means responsive to said comb filtered chrominance samples for developing a control signal proportional to the magnitude of said chrominance signal components;   signal splitting means coupled to said first filter and having first and second output terminals, said signal splitting means providing signals at its first and second output terminals which are mutually supplementary scaled replicas of the signals provided by said first filter, the amplitudes of said scaled replicas being a ratio proportional to the value of said control signal;   a second low-pass filter coupled to the first output terminal of said signal splitting means for further attenuating the chrominance signal components relative to the luminance signal components of the samples provided by said signal splitting means;   signal combining means coupled to said second filter and to the second output terminal of said signal splitting means for adding the samples provided thereby to produce samples representing a filtered output signal.   
     
     
       10. The apparatus set forth in claim 9, further comprising: means coupled to said signal combining means and to said comb filter for adding the samples provided by said signal combining means to the comb filtered luminance samples provided by said comb filter to produce samples representing the luminance signal components of said composite video signals.   
     
     
       11. The apparatus set forth in claim 10, further comprising: means coupled to said signal combining means and to said comb filter for subtracting the samples provided by said signal combining means from the comb filtered chrominance signals provided by said comb filter to produce samples representing the chrominance signal components of said composite video signals.   
     
     
       12. The apparatus set forth in claim 9 wherein said signal splitting means comprises: attenuating means coupled to said first filter and responsive to said control signal for providing, at said first output terminal, samples representing the samples provided by said first filter reduced in amplitude by an amount proportional to the magnitude of said control signal; and   subtracting means coupled to said first filter and to said attenuating means for providing, at said second output terminal, samples proportional to the difference between the samples provided by said first filter and the samples provided by said attenuating means.   
     
     
       13. The apparatus set forth in claim 9 wherein said control means comprises: a high-pass filter for providing samples representing the chrominance signal components to the relative exclusion of the luminance signal components of said comb filtered chrominance samples;   means coupled to said high-pass filter for providing separate sequences of samples each representing a respectively different one of the two color difference signal components of said chrominance samples; and   means responsive to said two color difference signal sample sequences for providing a control signal proportional to the greater of the instantaneous magnitude values of said two color difference signal samples.   
     
     
       14. The apparatus set forth in claim 9 wherein the frequency characteristic of said first low-pass filter is substantially the same as the frequency characteristic of said second low-pass filter.